Sound intensity recorder



i /"iarch 11, 1941. e. NEUMANN 2,234,573

SOUND INTENSITY RECORDER Original Filed Sept. 28, 1935 ffo/weys PatentedMar. 11, 1941 UNITED STATES PATENT OFFICE Original application September28, 1935, Serial No. 42,658, now Patent No. 2,178,844, dated November'7, 1939.

Divided and this application March I, 1939, Serial N0. 260,297. InGermany March 29. 1935 1 Claim.

This invention relates to a method and apparatus for the electricalmeasurement and recording of sound intensities.

In my Patent Number 2,178,641, dated Novemii her 7, 1939.01 which thepresent application is a divison, I have described a recording measuringinstrument which is especially adapted for the recording of soundintensities and in which the sound intensity is transformed into anelectrical output. The electrical output is fed to a potentiometer whichis continuously adjusted, by means of an electrical relay or coupling,under control of the potentiometer voltage, so that a constant voltageis produced at the secondary terminals of the potentiometer. Theposition of the potentiometer is then recorded as a measure of thearriving electric input. Thus it is possible to measure the objectivevalues of any sound intensities.

My present inventionhas for its object to provide means for measuringand recording the subjective valuesof sound intensities.

With this and other objects in view, as may become apparent from thewithin disclosures, the invention consists not only in the embodimentsherein pointed out and illustrated by the drawing, but includes furtherembodiments coming within the scope of what hereinafter may be claimed.

The character of the invention, however, may be best understood byreference to certain of its practical forms, as illustrated by theaccompanying drawing in which- Fig. 1 is a diagrammatic view showing asound recording apparatus having the invention applied thereto.

Fig. 1a is a detail-of Fig. 1, viewed at right angles thereto.

Fig. 2 is a modification of the potentiometer feeding circuit of Fig. 1.

Similar reference numerals denote similar parts in the diiferent views.Referring now to the drawing in greater detail, and first to Fig. 1, amicrophone i is exposed to the sound the intensity of which is to berecorded, and, together with the associated amplifier 2, makes up meansfor transforming sound into electrical output. A potentiometer 3 theresistance values of which along its length is preferably provided togrow in accordance with a logarithmic curve, receives its input currentfrom said sound transforming means, through the filter means I,-' II,III, IV which will be described later, and is provided with a slidingcontact or movable memher 4. One end of the potentiometer 3 and thesliding contact 4' are connected to the input tarminals of a secondamplifier 5, the output terminals of which are connected to the grid 6and thecathode l of the electron valve 8 acting as an audion valve, bymeans of the condensers and resistances necessary for the audioncircuit. In the anode circuit of the valve 8 there is inserted, inseries to the anode battery 9, a winding I0 forming part of the magneticcoupling. One terminal of the winding i0 is connected to the cathode Iof the audion 8, while the other terminal thereof is connected to thegrid ll of a secondelectron valve, in the anode circuit or which thesecond winding ll of the magnetic coupling is' inserted in series withan anode battery It. The cathodes of the two valves 8 and are connectedin parallel in the usual manner and are fed from the heating battery.IS. The magnetic coupling consists of two discs l6 and I! (see Fig. 1a)which are driven in the direction of the arrow, (Fig. l) and are made ofa suitable magnetisable or magnetic material, against the top and bottomedge of which the legs i8 and is, also consisting of a mangetic ormagnetisable material, of a fork 20 carrying the sliding contact 4 and apen 2i, lie with a slight resilient friction. The coils ill and it arestationarily arranged around the top and bottom part of the disc IS, in1 such a manner that the disc is free to rotate within the stationarycoils l0 and I4. The discs l6 and I! together with their axialconnection 22 and the legs I8 and IQ of the fork 20 form two closedmagnetic circuits for the fluxes generated by the coils l0 and H. Thefriction of one of the said legs-on the rotating discs can be increasedby energising the coils in the manner which will be described whilstthat of the other leg is decreased, so that the fork is moved by the Irotating discs to the right or left according to the energisation of thecoils. For the sake of clearness the driving arrangement for the discs[6 and I1 is not shown. A pen or stylus 2| mounted on the fork 20 drawsa curve on a paper strip or the like running over the rollers 23' and24, which curve represents the sound intensity values received by themicrophone. Thedriving arrangement for the paper stripis not shown, forthe sake of cleamess.

The operation of the said sound-recording device is as follows:

If no sound acts on the microphone I, the slidlng contact 4 and the pen2| may for instance be in their left hand end position, so that theinput terminals of the amplifier i are in connection with the endterminals of the potentiometer. In

this working condition the maximum anode current'fiows in the anodecircuit of the valve 8,

' which energises the coil 10 and consequently magnetises that part ofthe disc I 6 which happens to be the bottom part at the time. Thepotential drop obtaining in the coil I is impressed as a negative gridbias on the valve I 2, so that the anode circuit of the valve andconsequently also the excitation coil 14 is dead. By reason of this therotary discs I6 and I! of the magnetic coupling, which are magnetised intheir bottom parts only, tend to impart a movement to the fork 20 in aleft hand direction, while the fork cannot carry outthis movement as thesliding contact 4 and the pen 2| as mentioned above, are already intheir left hand end position, defined by a stop or the like, asindicated at 38.

Now if a sound of a definite intensity impinges on the microphone andconsequently generates currents in the microphone and thus a potentialon the potentiometer, the grid 6 becomes charged through the slidingcontact 4 and the amplifier 5,

which causes the anode current of the valve B'to drop, whereby theexcitation of the coil I0 is weakened and the potential of the grid ofthe second valve l2 altered, so that now a current is produced in-theanode circuit of this valve, which energises the coil l4 and therebymagnetises the toppart of the discs l6 and I1. Since the magnetisationof the lower disc parts is thus decreased and that of the top partsincreased, a push in the right hand direction is exerted on the fork 20which lasts for so long until, by reason of the action hereafterdescribed, the magnetisa-- tions created by the coils I0 and i4 balanceeach other in their effect on the legs l8 and I9. By

reason of the push to the right exerted on the fork 20 the slidingcontact 4 and thepen 2! are ly the anode current of the second valve l2and with it the energising current of the coil I4 is again weakened, andthese changes continue until the currents flowing in the coils Ill .andI4 balance each other in their actions on the fork 20, and the slidingcontact 4 and the pen 2'! assume their positions corresponding to thesound intensity, around which they execute only small pendu lousmovements as long as the sound intensity remains unaltered. If the soundvolume acting on the microphone decreases again, the anode current inthe anode circuit of the valve 8 rises so that the excitation of thecoil I0 is strengthened. Simultaneously the anode current of the valvel2 drops and with it the energising current of the coil l4 and a pushtowards the left is exerted on the fork so that it is re-adjusted to theleft, together with the sliding contact 4 and the pen 21, into theposition corresponding to the sound intensity now prevailing which'position will coincide with the left end position if a predeterminedminimum or zero sound volume acts upon the microphone.

Each time the sliding contact 4 and the pen 2| are in positionscorresponding to the sound volume received at the time, the forcesexerted by the coils Ill and H on the fork 20 neutralise each other.magnetic coupling and its energizing coils, this With a symmetricaldesign of the the fork 20 the currents in the anode circuits of 'thevalves 8 and I 2 have a definite relative intensity, and as the gridbias potential of the valve I2 is controlled only from the anode currentcircult of the valve 8, this proportion between the anode currents onlyoccurs atone definite value of the biassing potential of the grid 6,that is to say at one definite value of the input potential of theamplifier tapped or: by the sliding contact 4. I

It is therefore evident that thesliding contact 4 always takes up such aposition that the input potential of the amplifier 5 and the current andpotential proportions of the further circuit elements have the samevalues in a balanced condition. It will be understood that, if theterminals K of the amplifier 2 would be directly galvanically connectedto the terminals of the potentiometer, the objective sound intensityvalues would be recorded. Now, filter elements or circuits I, II, IIIare provided between the amplifier and the potentiometer, in order torecord the relative sound intensities as actually heard by humanpersons, this sound intensity being usually reis well known thesubjective intensity of sound varies from the objective sound intensityin a f manner depending from the frequency and the intensity of thesound. The correct representation of this dependency can be deductedfrom the Kingsbury curves (compare for instance the Handbuch derExperimentalphysik vol 17, Technische Akustik, part 1, 1934, page 134and following) or from the curves of Fletcher and Munson, (comp. AppliedAcoustics by Olson and Massa, Philadelphia, 1934, page 376) Therecording instrument shown in Fig. 1 is adapted for measuringelectrically subjective volumes of sound instead of objective volume ofsound by the fact that the potentiometer is provided with taps and thatthe tappings of the individual potentiometer sections are connected-with the sound transforming means through filters I, II, III, IV whichreduce the input fed to the potentiometer compared to the output of thetransforming means, by amounts varying in accordance with theintensities and frequencies of the sound and corresponding to thedifferences between the objective and subjective values of soundintensity at said difierent sound intensities and frequencies.

The logarithmic potentiometer 3 subdivided by means of tappings A1, 2,A:,;, A1, 4 into sections 31, 3g, 33, 34, and these'sections areprovided with filters I, II, III, IV which allowcentain definiteacoustic frequencies to pass or are provided with resistances dependingon the frequency, through which the input is fed to the sections. cultscomprise capacitiesx'ohmic resistances, potential dividers and orinductances in suitable combinations and are so calculated that thecharacteristic ofthe recording instrument corresponds to the abovementioned curves, so that it records the subjective instead of theobjective volume of sound. By means of providing a-suf- The cirficientlygreat number of tappings and filters this'.' I

desideratum can be attained to any desired degree of accuracy.

The Kingsbury curves deviate the more from the horizontal line, thesmaller the objective volume of the sound is. They have a downward bulging shape, which shows a minimum in a field of 1000-4000 Hertz and risesagain towards the limits of the acoustic frequency range. Consequentlythe filters must be so chosen that they filter the very high and thevery low acoustic frequencies but little, while the frequencies lying inthe field of from 1000-4000 Hertz are filtered more strongly, thisresult being attainable by correct dimensioning 01' the component partsof the filter, as will be readily understood by one skilled in the art.

In Fig. 1, the section 34 corresponds to the highest sound intensities,for which the characteristic is practically a straight horizontal line.Consequently the section 34 is connected to the terminals K through apotential divider S. The section 33 corresponds to a range of soundvolume for which the characteristic forms a curve which is but slightlybent downwards. Consequently the potential divider &, which togetherwith a part of the potential divider S4 connects the section 33 with theterminals K is fed through'a condenser C3. By these means the result isattained that the potential reaching the section 3: depends on thefrequency, and is the smaller the lower the frequency.

For the section 3: which corresponds to still lower sound volumes, astill more pronounced drop of the characteristic is attained by the factthat it is supplied with a potential from a potential divider S2, whichin its turn is connected through a condenser C: with the potentialdivider S: which is already supplied with a potential varying in thesame manner with the frequency.

Finally the section 31 is connected through a condenser C1 to a part ofthe potential divider S: and consequently has the most stronglydeclining characteristic.

Whereit is intended to consider also the rising parts of the Kingsburycurves in the highest frequency fields, inductance means may be insertedin series to the condensers C1, C2, C3 and such 7 an inductance is shownat D.

The shape of the characteristics obtained can be modified in any desiredmanner by the provision of ohmic resistances associated to individualcondensers or choking coils. Such a parallel resistance W is shown inconjunction with the condenser C1. I

Optionally, the potential dividers can be interconnected by condensersor choking coils in some suitable manner other than that shown inFig. 1. For example, in place of potential dividers, resistances R1, R2,R3 may be used through which one of the ends of each section isconnected with its condenser, whilst the other end thereof is suppliedwith a potential-through the adjacent section, such an arrangement beingshown in Fig. 2.

Moreover, it is possible to connect the choking coils or'condensers inparallel to sections of the potential dividers or resistances.

I am aware that many further changes may be made andnumerous details ofconstruction may be varied through a wide range without departing fromthe principles of this invention, and I, therefore, do not purposelimiting the patent granted hereon otherwise than is necessitated by theprior art.

I claim:

In an apparatus for recording the subjective values oi! soundintensities, a potentiometer connected to the terminals of a soundtransforming means of such apparatus and including a plurality ofindividual sections receiving their input from said transforming means,and corresponding to various ranges of sound intensities, a plurality ofpotential dividers electrically connecting said sections to saidterminals, at least one of said sections being connected through one ofsaid dividers and a part or another, a condenser in circuit with theformer of said-dividers, and an inductance in series with said condenserand connected to one of said terminals.

onona 1

